A device having a two-way radio communication function such as a cellular phone, and a radio communication system using this type of device as a communication terminal provide a difference between a frequency of a transmission radio wave and a frequency of a reception radio wave and separate a transmission signal and a reception signal in a duplexer using the above frequency difference so that they can conduct transmission/reception using one antenna included in the communication terminal.
A duplexer is formed of a receiving side filter that filters (selects a frequency of) a reception radio wave and outputs the filtered radio wave to a reception processing unit in a device, and a transmitting side filter that filters (selects a frequency of) a transmission signal from a transmission processing unit in the device to output the filtered signal to an antenna, and has a center frequency different from that of the receiving side filter. Some of these filters employ a ladder-type filter in which small-sized and low-loss elastic wave resonators, which are, for example, SAW (Surface Acoustic Wave) resonators, are connected in a ladder form. Conventionally, these ladder-type filters are formed in different chips respectively, and the respective chips are combined on a substrate, and thereby a duplexer is formed.
In contrast to this, in accordance with recent further miniaturization of the cellular phone or the like, a smaller-sized duplexer has also been required, and as one means for the above, the present inventor has studied that the two ladder-type filters are stored in one chip to be modularized and to be formed into one chip.
For example, FIG. 13 shows a structure example of a duplexer 100 in which two ladder-type filters are formed into one chip. The duplexer 100 has a low band side filter 3 being a filter on a transmitting side and a high band side filter 4 being a filter on a receiving side integrally provided on a module substrate 10 constituting a base of a chip with respect to a single antenna port 2. In this example, the low band side filter 3 is formed on a square-shaped piezoelectric substrate 30 disposed in a region on the left side of a center of the module substrate 10, and the above piezoelectric substrate 30 is made of, for example, LiTaO3, LiNbO3, quartz crystal, or the like. When in FIG. 13, an upper side is set to the front and a lower side is set to the rear at present, in a half region of the right side on the piezoelectric substrate 30, three series arms 31a to 31c composed of SAW resonators are disposed in one line from the rear to the front, and in a region of the left side, two parallel arms 32a, 32b composed of SAW resonators similarly are disposed in one line from the rear to the front. Then, the respective series arms 31a to 31c are coupled in series by a connecting line 331, and further the parallel arm 32a, 32b is connected in parallel between the series arms 31a to 31c, and thereby a T-type five-stage ladder-type filter is formed.
It is designed in a manner that an input portion of a transmission signal to the series arm 31a is connected to a low band side filter port 6 provided on a left corner of a rear end of the module substrate 10 by a bonding wire 311, and a transmission signal from a not-illustrated transmission processing unit is input to the low band side filter 3. Further, an output portion of the transmission signal from the series arm 31c is connected to the antenna port 2 provided on a center portion of a tip side of the module substrate 10 by the bonding wire 311, and downstream sides of the respective parallel arms 32a, 32b are connected and grounded to ground ports 341, 342 provided on a left corner of the module substrate 10 in an arranged manner by the bonding wires 311 respectively.
Here, the respective SAW resonators 31a to 31c, 32a and 32b are composed of the previously described piezoelectric substrate 30 and electrode parts 8 patterned on the above piezoelectric substrate 30, and each of the electrode parts 8 includes an IDT (Interdigital Transducer) electrode 81 and grating reflectors (that will be called reflectors, hereinafter) 82 disposed on the right and left of the IDT electrode 81, which are shown in FIG. 14(a). Note that in the respective drawings to be explained hereinafter such as FIG. 13, there is sometimes a case that the IDT electrode 81 and the reflectors 82 are appropriately illustrated in a simplified manner as shown in FIG. 14(b).
On the other hand, the high band side filter 4 is formed in a ladder-type filter in which series arms 41a to 41c and parallel arms 42a to 42b composed of SAW resonators respectively are coupled to form a T-type circuit similarly to the previously described low band side filter 3. Detailedly, the high band side filter 4 has a piezoelectric substrate 40 different from the piezoelectric substrate 30 on a low band side filter 3 side provided in a region of the right side of the module substrate 10, and on the above piezoelectric substrate 40, the three series arms 41a to 41c are disposed from the front to the rear, and the two series arms 41a, 41b out of them are disposed in a half region of the left side of the piezoelectric substrate 40 and further the remaining single series arm 41c is disposed in a half region of the right side of the piezoelectric substrate 40 in two lines. Further, the three parallel arms 42a to 42c are disposed in two lines from the front to the rear so that the two parallel arms 42a, 42b on the front side are disposed in the half region of the right side of the piezoelectric substrate 40 and the remaining single parallel arm 42c is disposed in the half region of the left side of the piezoelectric substrate 40.
These series arms 41a to 41c are connected in series by a connecting line 431 one another, and the parallel arm 42a to 42c is connected in parallel between the series arms 41a to 41c and connected in parallel to the series arm 41c at a rear stage, and thereby a T-type six-stage ladder-type filter is formed. The reason why the number of stages of the parallel arms on a high band side filter 4 side is increased by one stage in this example is that an attenuation of the high band side filter 4 with respect to a low frequency signal is increased to prevent the low frequency signal from flowing into a reception processing unit side.
Then, it is designed in a manner that an inlet side of a reception signal in the series arm 41a is connected to the previously described antenna port 2 by a bonding wire 411, and an outlet side of the reception signal from the series arm 41c is connected to a high band side filter port 7 provided on a right corner of the rear end of the module substrate 10 by the bonding wire 411, and then the reception signal that has passed through the high band side filter 4 can be output to a not-illustrated reception processing unit. Further, outlet sides of the three parallel arms 42a to 42c are connected and grounded to ground ports 441 to 443 provided on a right corner of the module substrate 10 from the tip to the rear in an arranged manner by the bonding wires 411 respectively.
Here, the respective bonding wires 311, 411 on the low band side filter 3 side and the high band side filter 4 side form inductors for adjusting positions of low band side pass frequency bands 104, 105 of respective low band side filter characteristics 101, 102. Further, for easier identification, in each of the drawings to be explained hereinafter, an identification code of “s” is appropriately attached to the series arms 31a to 31c, 41a to 41c, and an identification code of “p” is appropriately attached to the parallel arms 32a and 32b, 42a to 42c. 
The two filters 3, 4 are formed into one chip, thereby making a region where, for example, a chip cover is provided in common to enable miniaturization of the duplexer 100 to be achieved. However, when the duplexer 100 shown in FIG. 13 is well seen, for example, a cutting margin “a1” is provided to the right of the series arms 31a to 31c disposed in the region of the left side of the low band side filter 3. The above cutting margin is provided for the reason that when the electrode parts 8 and the connecting line 331 are patterned to form a large number of the filters 3 on, for example, a piezoelectric wafer and each of the filters 3 is cut off from the above wafer, damage to the electrode parts 8 and the like formed on the wafer is prevented, and is set to, for example, “a1=50 μm” or so.
Such a cutting margin is similarly provided also in the high band side filter 4, (which is described as “a2”), and even though the low band side filter 3 and the high band side filter 4 are disposed to make a distance “b” between, for example, the two piezoelectric substrates 30 and 40 approach to several μm or so, a distance (a1+a2+b) of 100 μm or more is needed between the two filters 3 and 4, which results in a cause of preventing further miniaturization of the duplexer 100. Thus, the present inventor forms a duplexer 100a by patterning the low band side filter 3 and the high band side filter 4 on a common piezoelectric substrate 11 as shown in FIG. 15, thereby omitting cutting margins between these filters 3 and 4 to achieve further miniaturization of the duplexer 100a. 
However, when the duplexer 100a shown in FIG. 15 is manufactured and a frequency characteristic thereof is measured, a deterioration in a characteristic, which is a later-described isolation characteristic concretely, of the duplexer 100a is confirmed as compared with the conventional duplexer. Thus, the cause in which such a deterioration is seen is pursued, and then it is found that the low band side filter 3 and the high band side filter 4 are formed on the common piezoelectric substrate 11, and thereby as shown by wavy line arrows in FIG. 15, SAWs leaked from the reflectors 82 reach the resonators adjacent to each other in a main propagation direction of SAWs, (which are, for example, between the series arm 31a and the parallel arm 42c, and between the series arm 31c and the series arm 41a in FIG. 15), in the low band side filter 3 and the high band side filter 4, resulting that the deterioration in the characteristic of the duplexer 100a is caused. Here, the main propagation direction of SAWs in the present invention is set to a propagation direction of SAWs matched to a direction in which respective electrode fingers 83 of the IDT electrode 81 are provided continuously as shown by arrows in FIG. 14(a).
In order to prevent interference as above, it is also possible to consider a method in which the respective resonators 31a to 31c, 32a, 32b and the respective resonators 41a to 41c and 42a to 42c are disposed in an offset manner not to be adjacent to each other in the main propagation direction of SAWs. However, there are also many cases that it is difficult in layout to dispose the respective resonators on the piezoelectric substrate 11 miniaturized as described previously in an offset manner. Further, in order to reduce a leakage amount of a SAW, a method to widen a width of the reflector 82 can also be considered. However, each of the reflectors 82 plays a role of again reflecting a SAW propagated to the side of the IDT electrode 81 to the IDT electrode 81, so that in the case when a width of the reflector 82 on one side of the reflectors 82 provided on the right and left of the IDT electrode 81 is widened, a width of the reflector 82 on the other side also has to be similarly widened not to deteriorate characteristics of the filters 3, 4, which results in a cause of preventing miniaturization of the duplexer 100a. 
Here, in Patent Document 1, there has been described a duplexer on which SAW resonators formed in a manner that in order to reflect a SAW to leak in a main propagation direction from a reflector and a SAW to leak in a direction inclined relative to the above main propagation direction, four or six of additional reflectors are provided in directions in which these SAWs leak, are mounted. However, Patent Document 1 in which the duplexer is described does not explain that in the case in which disposition relationship the two SAW resonators are, the SAW leaked from the reflector causes a deterioration in an isolation characteristic of the duplexer. Thus, four or six of the additional reflectors are provided in all the SAW resonators, thereby increasing a disposition area of each of the SAW resonators, and the existence of these additional reflectors prevents further miniaturization of the duplexer.
Further, the art described in Patent Document 1 employs mounting in a flip chip system, and is constituted in a manner that an annular electrode is provided around the periphery of each of a low band side filter and a high band side filter, and parallel arms are connected to the above annular electrode to be grounded. In contrast to this, in the case when mounting by, for example, a bonding wire system is employed, a method of how the duplexer 1 is miniaturized is not disclosed.
[Patent Document 1]
Japanese Patent Application Laid-open No. 2007-97117: paragraph 0065, paragraphs 0075 to 0078, FIG. 2, FIG. 5